Allow returning text/plain Content-Types as 'data' output argument.
So far, all the esim/http_json_api functions require a JSON response.
However, a specific vendor has a list function where the request is JSON
but the response is text/plain CSV data. Allow and return in a dict.
Change-Id: Iba6e4cef1048b376050a435a900c0f395655a790
a4b5841f78